Due to their small size, high Q values, and very low insertion losses at microwave frequencies, particularly those above 1.5 Gigahertz (GHz), Bulk Acoustic Wave (BAW) filters have become the filter of choice for many modern wireless applications. In particular, BAW filters are the filter of choice for many 3rd Generation (3G) and 4th Generation (4G) wireless devices, and are destined to dominate filter applications for 5th Generation (5G) wireless devices. For mobile devices, the low insertion loss of the BAW filter provides many advantages such as improved battery life, compensation for higher losses associated with the need to support many frequency bands in a single mobile device, etc.
One example of a conventional BAW resonator 10 is illustrated in FIG. 1. In this example, the BAW resonator 10 is, in particular, a Solidly Mounted Resonator (SMR) type BAW resonator 10. As illustrated, the BAW resonator 10 includes a substrate 12, a reflector 14 over the substrate 12, and a transducer 16 over the reflector 14. The reflector 14 is typically formed by a stack of reflector layers (not shown), which alternate in material composition to produce a significant reflection coefficient at the junction of adjacent reflector layers. The transducer 16 includes a piezoelectric layer 18, which is sandwiched between a top electrode 20 and a bottom electrode 22. The transducer 16 may also include a top electrode lead 24 and a bottom electrode lead 26. The top electrode lead 24 resides on the piezoelectric layer 18 and is connected to the top electrode 20. The bottom electrode lead 26 resides underneath the piezoelectric layer 18 and is connected to the bottom electrode 22.
The BAW resonator 10 is divided into an active region AR and an outside region OR. The active region AR is electrically driven and generally corresponds to the section of the BAW resonator 10 where the top and bottom electrodes 20 and 22 overlap and also includes the layers below the overlapping top and bottom electrodes 20 and 22. The outside region OR is not electrically driven and corresponds to the section of the BAW resonator 10 that surrounds the active region AR.
In operation, acoustic waves in the piezoelectric layer 18 within the active region AR of the BAW resonator 10 are excited by an electrical signal applied to the bottom and top electrodes 20 and 22. The frequency at which resonance of the acoustic waves occurs is a function of the thickness of the piezoelectric layer 18 and the mass of the bottom and top electrodes 20 and 22. At high frequencies (e.g., greater than 1.5 GHz), the thickness of the piezoelectric layer 18 may be only micrometers thick and, as such, the BAW resonator 10 is fabricated using thin-film techniques.
In order to protect BAW resonators or BAW devices from external elements (such as moisture, contamination, etc.), housings, also known as Wafer-level packaging (WLP) walls and caps, are applied to enclose the BAW resonators or BAW devices. However, during the lithography process, cross-linking, also known as “scumming,” of the WLP walls may be present in regions where top electrode leads, with a high reflectance top layer, are routed under the WLP walls. These top electrode leads cause reflection or scattering of light to the WLP walls over them, which results in scumming of the WLP walls.
FIG. 2 shows a top view of a BAW device 28 with scumming effects. As illustrated, the BAW device 28 includes a first BAW resonator 30, a second BAW resonator 32, and a WLP enclosure 34. A first top electrode 36 of the first BAW resonator 30 and a second top electrode 38 of the second BAW resonator 32 are formed on a piezoelectric layer 40 of the BAW device 28. The WLP enclosure 34 is coupled to the piezoelectric layer 40 to encapsulate the first top electrode 36 and the second top electrode 38 separately. In addition, a first top electrode lead 42 extends from the first top electrode 36 and is routed under a first portion of an outer wall of the WLP enclosure 34. A second top electrode lead 44 extends from the second top electrode 38 and is routed under a second portion of the outer wall of the WLP enclosure 34. Herein, bottom electrodes (now shown) of the first BAW resonator 30 and the second BAW resonator 32 may be connected by a bottom electrode lead 46, which is underneath the piezoelectric layer 40 and routed under an inner wall of the WLP enclosure 34.
Notice that the bottom electrode lead 46 is separate from the inner wall of the WLP enclosure 34 by the piezoelectric layer 40. Even with a high reflectance top layer, the bottom electrode lead 46 will not cause scumming of the inner wall of the WLP enclosure 34. However, since the first/second top electrode lead 42/44 is adjacent under the outer wall of the WLP enclosure 34, when the first/second top electrode lead 42/44 includes a high reflectance top layer, scumming 48 is present in the first/second portion of the outer wall of the WLP enclosure 34. The scumming 48 may extend over the first top electrode 36 and the second top electrode 38, which will cause significant yield loss of the first and second BAW resonators 30 and 32.
Accordingly, there remains a need for improved BAW device designs to inhibit scumming of the WLP walls. Further, there is also a need to keep the final product size effective and cost effective.